As gas turbine engine technology advances and engines are required to be more efficient, gas temperatures within the engines continue to rise. However, the ability to operate at these increasing temperatures is limited by the ability of the superalloy turbine blades and vanes to maintain their mechanical strength when exposed to the heat, oxidation, and corrosive effects of the impinging gas. One approach to this problem has been to apply a protective thermal barrier coating that insulates the blades and vanes and inhibits oxidation and hot gas corrosion.
Typically, thermal barrier coatings are applied to a superalloy substrate and include a bond coat and a ceramic top layer. The ceramic top layer is applied either by the process of plasma spraying or by the process of electron beam physical vapor deposition (EB-PVD). Use of the EB-PVD process results in the outer ceramic layer having a columnar microstructure. Gaps between individual columns allow the coating to expand and contract laterally upon thermal cycling without developing stresses that could cause spalling. Strangman, U.S. Pat. Nos. 4,321,311, 4,401,697, and 4,405,659 disclose thermal barrier coatings for superalloy substrates that contain a MCrAlY layer, an alumina layer, and an outer columnar grained ceramic layer. A more cost effective system is disclosed in Strangman U.S. Pat. No. 5,514,482, which teaches a thermal barrier coating for a superalloy substrate that contains an aluminide layer, an alumina layer, and an outer columnar ceramic layer. A low conductivity system is disclosed in Strangman U.S. Pat. No. 6,482,537. A thermal barrier coating for turbine engine components is disclosed in Tolpygo U.S. Pat. Appl. Pub. No. 2010/0327213, the disclosure of which is incorporated by reference herein in its entirety.
The ceramic layer is commonly zirconia stabilized with yttria. The prior art teaches that the amount of yttria can range from about 6 percent to about 35 percent of the layer. (See U.S. Pat. Nos. 5,238,752 and 4,321,310). It is also known in the prior art that cubic zirconia, which is zirconia stabilized with more than about 15 percent yttria, has lower thermal conductivity relative to tetragonal zirconia which is stabilized with about 6 to about 8 percent yttria. However, despite the disadvantage of higher thermal conductivity, most commercially available thermal barrier coatings use tetragonal zirconia stabilized with 7 to 8 weight percent yttria for the ceramic layer because it is more reliable due to its superior capability to resist spalling and particulate erosion.
The prior art further teaches a thermal barrier coating having a ceramic layer that has thermal conductivity less than or equal to that of cubic zirconia and resistance to spalling of tetragonal zirconia as well as a need for a method to make such a coating. (See U.S. Pat. No. 6,482,537). These advanced coatings contain other metal oxides in addition to zirconia and yttria. However, such systems have been found to be difficult to deposit on the superalloy substrates using EB-PVD process, leading to complicated coating recipes. The multi-component coatings often become locally enriched with or depleted by individual components (constituents), leading to chemistry variation within the coating, more specifically, across its thickness. Such chemical instability may adversely affect TBC properties and performance, for example, by inducing thermal expansion mismatch between coating layers having different compositions, or creating a particular combination of components susceptible to destructive phase transformation during service.
Accordingly, it is desirable to provide low conductivity thermal barrier coatings that do not suffer chemistry variations within the coating. More particularly, it is desirable to provide low conductivity thermal barrier coatings wherein their individual constituents maintain consistent concentrations across the coating thickness. This is especially important when the coating is deposited using EB-PVD process with all coating constituents being evaporated from a single source. Such a “single-source” method is more practical in comparison with other, more complicated coating recipes involving multiple evaporation sources. However, evaporation of several different oxides simultaneously from a single source often results in substantial variations of the coating composition because the composition of the vapor phase does not always remain stable during deposition. As such, it would be desirable to reduce compositional variations in EB-PVD TBCs. Furthermore, other desirable features and characteristics of the disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawing and this background of the disclosure.